1. Field of the Invention
The present invention relates to an electro-optical plasma addressing device having a two-layer construction, consisting of an electro-optical cell, such as a liquid crystal cell, and a plasma cell and, more specifically, to the electrode construction of the plasma cell of an electro-optical plasma addressing device.
2. Description of the Related Art
An active matrix addressing system is a generally known means for enhancing the resolution and contrast of an electro-optical device of a matrix type employing a liquid crystal cell as an electro-optical cell, such as a liquid crystal display. The active matrix addressing system employs switching elements, such as thin-film transistors or the like, in combination with the picture elements and drives the switching elements in linear sequence. Since the active matrix addressing system needs a substrate provided with a plurality of semiconductor elements, such as thin-film transistors, the percentage of acceptable electro-optical optical devices of a matrix type decreases when the size of the display panel is increased.
A means for eliminating such a disadvantage of the electro-optical device, disclosed in Japanese Patent Laid-open (Kokai) No. 1-217396 (corresponding to U.S. Pat. Nos. 4,896,149 and 5,077,553) employs plasma switches instead of the switching elements, such as thin-film transistors. The construction of a plasma addressing display employing plasma switches for driving a liquid crystal cell will be briefly described hereinafter.
Referring to FIG. 6, the plasma addressing display has a laminate flat panel construction comprising a liquid crystal cell 101, a plasma cell 102 opposed to the liquid crystal cell 101, and an intermediate plate 103 held between the liquid crystal cell 101 and the plasma cell 102. The plasma cell 102 has a glass substrate 104 provided in its surface with a plurality of parallel grooves 105 extending, for example, along the rows of a matrix. The grooves 105 are covered closely with the middle plate 103 to form separate plasma chambers 106. An ionizable gas is sealed in the plasma chambers 106. Ridges 107 between the adjacent grooves 105 serves as partition walls separating the plasma chambers 106 from each other and as gap spacers spacing apart the plasma chambers 106. A pair of parallel electrodes 108 and 109 are extended on the bottom surface of each groove 105 for ionizing the gas sealed in the plasma chamber 106 to produce a discharge plasma. The electrodes 108 are anodes A, and the electrodes 109 are cathodes K. Such discharge regions are row scanning units.
The liquid crystal cell 101 has a transparent substrate 110 disposed opposite to the intermediate plate 103 with a specified gap therebetween and the gap is filled up with a liquid crystal 111. Signal electrodes D of a transparent conductive material are formed on the inner surface of the transparent substrate 110. The signal electrodes D extend perpendicularly to the axes of the plasma chambers 106 to function as column driving units. The virtual intersections of the row scanning units and the column driving units form a matrix of picture elements.
The operation of the plasma addressing display of FIG. 6 will be described briefly with reference to FIG. 7.
An external driving circuit for driving the plasma addressing display comprises a signal circuit 201, a scanning circuit 202 and a control circuit 203. The signal electrodes Dl to Dm, which function as column driving units, are connected respectively through buffers to the signal circuit 201, the cathodes Kl to Kn are connected respectively through buffers and resistors R to the scanning circuit 202, and the anodes Al to An are connected to a common ground. The scanning circuit 202 scans the cathodes Kl to Kn in linear sequence, and the signal circuit 201 applies analog image signals to the signal electrodes Dl to Dm in synchronism with the scanning operation of the scanning circuit 202. The control circuit 203 controls the signal circuit 201 and the scanning circuit 202 for synchronous operation. A plasma discharge region, i.e., a row scanning unit, is formed along the selected cathode Kj (j=1, 2, . . . , n-1 or n). The virtual intersections of the row scanning units and the column driving units form picture elements 204.
Load resistors R are connected respectively to the cathodes Kl to Kn to secure uniform plasma discharge over the entire area of the screen by suppressing difference in discharge current between the row scanning units. If an excessive discharge current is supplied to the cathode Kj, a voltage drop according to the current occurs across the corresponding loading resistor R to suppress the variation of effective voltage applied to the cathode Kj.
Referring to FIG. 8 showing two picture elements 204 among those of the plasma addressing display of FIG. 7, each picture element 204 is formed by connecting a sampling capacitor consisting of the signal electrode Dl (D2) and the liquid crystal 111 filling the gap between the transparent substrate 110 and the intermediate plate 103, and a plasma sampling switch S in series. The function of the plasma sampling switch S is equivalent to that of the discharge region, i.e., the row scanning unit. When activated, the interior of the discharge region is kept substantially generally at an anode potential. After plasma discharge has terminated, the potential of the discharge region coincides with a floating potential. An analog image signal is applied through the sampling switch S to the sampling capacitor of the picture element 204 for sampling hold.
Whereas the known plasma display panel forms a point-like discharge region, this plasma addressing display addresses the picture elements addressed through the plasma sampling switches. In this plasma addressing display, a discharge region, i.e., the row scanning unit, is formed between the paired anode and cathode. The length of this discharge region is considerably long when the plasma addressing display has a large screen and local discharge occurs to form a nonuniform row scanning unit due to voltage drop caused by the resistances of the anode and the cathode. If a nonuniform row scanning unit is formed, the resistances of the plasma sampling switches S differ from each other. Although the loading resistor R is connected to each cathode to produce uniform plasma discharge in the row scanning unit, the loading resistor R is unable to suppress the occurrence of local plasma discharge. Accordingly, a sufficiently high current must be supplied to the row scanning unit to produce stable plasma discharge over the entire length of the row scanning unit. Consequently, the contrast of the image displayed on the plasma addressing display is reduced by unnecessary plasma, power consumption is increased and the life of the plasma addressing display is shortened. Thus, when plasma discharge in the row scanning unit having a large length is controlled by the conventional discharge current compensating system, the distribution of plasma discharge is liable to vary delicately and the local variation of the distribution of plasma discharge cannot be absorbed and, therefore, the picture elements differ unavoidably from each other in brightness.